HTHSCI 1DT3 Lecture 6: 1.1 Cells.4
23 Jun 2018
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Once generated, cortical neurons move away from the ventricular zone towards the outer
surface to lodge in the cortical plate. The cortical plate is the outer regions of the brain (i.e.
cortex).
The cortical plate develops from inside out, and neurons use the radial glia as a guide (long
elongated pinkish-red cells spanning ventricular zone to marginal zone). The earliest generated
neurons (lighter green) reach first, and as the cortical plate develops, newer neurons (darker
green) pass the earlier ones and develop the external layers ‘higher up’ in the brain.
The cortical plate refers to the outer regions of the brain (i.e. cortex), and earlier on in
development, the neurons in light green migrate up using the radial glia as a guide. As
development progresses, newer neurons (dark green) migrate up past the older neurons (light
green), and are positioned further outwards.
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After the neurons migrate to their final positions, they become polarized and extend an axon
towards their target tissues and form synapses (see later lectures). They also develop dendrites at
this point.
Neurons are considered post-mitotic, and once they differentiate into neuronal morphology,
they cannot re-enter mitosis. Therefore, neurogenesis does not continue throughout life (unlike
most other cells of the body).
Therefore, neurons have to last a lifetime, and death/damage to neurons have severe
consequences. They therefore need to be resilient, adapt to longevity and be supported by other
cells.
Neuron Theory: Neurons are discrete elongated cells that communicate with each other via
synapses (Suggested by Santiago Ramon y Cajal, proved Golgi was wrong – who suggested that nerves
formed a syncytium network which was fused together).
Morphology:
Elongated morphology, and very large volume and surface area
Broadly specialized regions:
Soma – (cell body) containing nucleus
Dendrites – receives incoming synaptic transmission from other neurons
Axon – transmits action potentials to target tissue or neurons
The distal tip of a neuron will differ depending on whether the neuron is mature or
developing. If it is developing, the tip of the axon will be a growth cone, rather than a pre-
synaptic terminal (and growth cones can change their shapes according to guidance cues)
Further specialized regions include:
Axon hillock (aka – initial segment)
Myelin sheath
Nodes of Ranvier
Pre-Synaptic terminals
Post-Synaptic specialisations (on dendrites, e.g. dendritic spines)
Myelin
Myelin is formed by oligodendrocytes (CNS) and Schwann Cells (PNS), which wrap around
the axon.
Myelin is rich in specialized lipids, giving it its white appearance (hence ‘white matter’)
Myelin has two main roles:
Protection – of axons (e.g. from toxins)
Fast Conductance – (main role), allows fast transmission of action potentials
Only axons are myelinated, but not all axons are myelinated (e.g. cerebellar granule neurons,
and nociceptive sensory neurons are unmyelinated).
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Neuronal Diversity: Neurons can be classified in multiple ways
Degree of myelination: myelinated (e.g. motor neurons) vs. unmyelinated (e.g. cerebellar
granule neuron)
Direction of signal / Type of target:
Interneuron – in the CNS synapsing with another neuron (e.g. cerebellar granule
neuron)
Efferent CNS Neuron – transmitting action potentials AWAY from the CNS (e.g. motor
neuron)
Afferent PNS Neuron – transmitting action potentials TOWARDS the CNS (e.g. sensory
neuron)
Location of Cell Body:
CNS neurons with cell bodies in the brain or spinal cord (e.g. motor neuron)
PNS neurons with cell bodies outside brain or spinal cord (e.g. sensory neurons in
dorsal root ganglion)
Morphology / Number of Projections from Cell Body:
Unipolar (e.g. sensory neuron) with only a single projection from the cell body
Bipolar (with a single axon and single dendrite – e.g. bipolar retinal neurons)
Multipolar (single axon and multiple dendrites – e.g. pyramidal neuron, Purkinje
neuron, motor neuron)
Synaptic Function
The function of a neuron is to transmit and receive action potentials, or stimulate electrical
activity in a target tissue such as skeletal or cardiac muscle.
When action potentials reach the synapse, neurotransmitters released by a presynaptic terminal
bind to postsynaptic receptors of the neuron/muscle it is synapsing with.
Some neurotransmitters reduce the likelihood of action potential firing by its target
(postsynaptic neuron), which are called inhibitory neurotransmitters which decrease
depolarization (e.g. GABA, dopamine)
Other neurotransmitters increase the likelihood of potential firing by its target neuron, and
these are called excitatory neurotransmitters (increase depolarization) (e.g. glutamate,
acetylcholine)
Glutamergic and GABAergic synapses differ morphologically (See diagram below)
Synapses can form on dendritic shafts, cell bodies or on dendritic spines (though not all
neurons have dendritic spines)
Synapses are important structures, with a distinctive mushroom-like appearance. Note how
weaker synapses are present in mentally retarded patients (as shown below).
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Document Summary
Once generated, cortical neurons move away from the ventricular zone towards the outer surface to lodge in the cortical plate. The cortical plate is the outer regions of the brain (i. e. cortex). The cortical plate develops from inside out, and neurons use the radial glia as a guide (long elongated pinkish-red cells spanning ventricular zone to marginal zone). The earliest generated neurons (lighter green) reach first, and as the cortical plate develops, newer neurons (darker green) pass the earlier ones and develop the external layers higher up" in the brain. The cortical plate refers to the outer regions of the brain (i. e. cortex), and earlier on in development, the neurons in light green migrate up using the radial glia as a guide. As development progresses, newer neurons (dark green) migrate up past the older neurons (light green), and are positioned further outwards.
